1. Field of the Invention
The present invention relates to a plant control device for determining a plant input of operation to control a certain plant to a target value, and more particularly applies the principle of the plant control device to a clutch slip control device and an idling speed control device for an internal combustion engine. Furthermore, the present invention relates to a control which provides a control in conformity with a predetermined operational characteristics and even upon a perturbation of the operation characteristics exerts high-speed controllability of a controlled variable for the object to be controlled, in specific, clutch slip conditions or idling engine speed. The invention also pertains to a method of controlling a certain plant and more specifically to a method of controlling clutch slip conditions and a method of controlling the idling engine speed.
2. Description of the Related Art
With recent advancement in control technology, a variety of control devices have been proposed to allow stable and high-speed controls of various plants or objects to be controlled in a vehicle. For example, there are a variety of known clutch slip control devices including those for controlling slip conditions of a lock-up clutch of a torque converter. Such slip control devices are designed to solve contradictory problems; that is, direct connection of inputs with outputs of the torque converter transmits the vibrations of an engine directly to a transmission in a lower engine speed range, thus worsening the riding comfortability, whereas disconnection of inputs and outputs in a wide range of engine speed prevents sufficient reduction of the fuel consumption rate.
Improvement has conventionally been given to these slip control devices to reconcile the high response and control stability. One example of proposed improvement includes a process of calculating a current plant input based on a deviation of an actual slip revolution speed from a target slip revolution speed as well as a differential and an integral of the deviation or a differential and a second differential of the deviation (JAPANESE PATENT PUBLICATION GAZETTE No. H2-586). Another example includes a process of expanding these quantities in times series to calculate an increase in the plant input (JAPANESE PATENT LAYING-OPEN GAZETTE No. S64-30966). By matching the characteristics of the control devices to those of a plant or slip adjusting mechanism, these improved control devices can stably regulate and maintain the actual slip equal to or close to a target value and realize the high response to the target value without lowering the stability.
Other systems for compensating perturbations of characteristics of a plant have also been proposed. The operation characteristics of the plant vary with a variation in driving conditions of an internal combustion engine. A known device disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. S60-143268 detects the loading condition of the internal combustion engine and varies a control constant of slip control according to the detected loading condition. When the characteristics of the control system vary with a variation in the operating conditions, the conventional technique detects a quantity of state causing or representing the characteristic perturbation and varies the control constant according to the quantity of state, so as to maintain the sufficient control performances over a wide operation domain.
These conventional control devices have the following drawbacks:
(1) The system for varying the control constant against the characteristic perturbation of the control system generally stores the control constant as a map. A large map is, however, required to compensate for various characteristic perturbations. Such a large map requires a relatively large storage capacity, while a control device mounted on a vehicle has only a limited storage capacity for storing various control programs and data.
(2) A system for selecting one among a plurality of control constants causes a plant input to discretely vary with respect to the plant. The discrete variation in plant input results in abrupt change of the controlled variable of the plant. A possible technique to prevent the discrete variation in plant input prepares and stores a number of maps of the control constants, and interpolates the control constant every time when a different control constant is selected. This technique, however, undesirably increases the storage capacity for the maps, and moreover such interpolation may result in insufficient control performances to make the control system unstable.
(3) Another proposed technique interpolates the plant input instead of the control constant. This technique calculates a plurality of plant inputs based on the respective maps of control constants in the process of selecting a control constant, determines a weighting average of the plural plant inputs, and outputs the interpolated plant input to the control system. Calculation of the plural plant inputs takes a relatively long time, which may be greater than a sampling period. This method does not use raw data of a plant input corresponding to a specific control constant, but outputs a mathematically processed plant input to the control system. This may result in unpreferable control characteristics.
(4) These conventional devices for or methods of controlling a system where characteristics vary with time or according to the operating conditions of the plant lay stress on the stability of the control system and can accordingly not realize the high response of the control system. These conventional systems can not attain the sufficient control under any transient conditions.
These problems are found in various devices mounted on the vehicle. For example, operation characteristics of a lock-up clutch of a torque converter significantly vary with the rotating speed of its turbine. A known control device for controlling a slip of the lock-up clutch can not realize stable and high-speed slip control over the perturbed characteristics due to a significant variation in rotating speed. When there is a significant difference in properties between individual lock-up clutches and slip-regulating hydraulic control systems or when deterioration of frictional material or operating oil varies the frictional characteristics of the lock-up clutch or .mu.-v characteristics of the clutch from the original design conditions to damage the stability of the slip revolution speed, the conventional control device may not stably or rapidly control the slip equal to or close to the target value. It is, however, not practical to apply the method of selecting one of the plural control constants according to the difference between the individual control systems or the time-based variation of the control system, since this conventional technique has the drawback described above.
Similar problems are also observed in an idling speed control device due to a time-based variation in valve characteristics as well as a variation in quantity of intake air consumed for idling accompanied with a change in negative pressure of the intake manifold or other driving conditions of the internal combustion engine. The internal combustion engine is not driven at a fixed speed or fixed loading but is operated over a wide dynamic range. This naturally causes a variation in operating conditions of the control system and thereby a variation in control characteristics. A fuel injection control device and an ignition timing control device (or the corresponding methods) accordingly have the problems mentioned above. Similar problems are also observed in a transmission controller, a traction controller, and an anti-skid control device (or the corresponding methods) whose control characteristics vary with a variation in driving range of the vehicle or road surface conditions.